Microwave cavity resonator with printed circuit interior walls and modulation coils

ABSTRACT

A microwave cavity resonator useful in electron paramagnetic resonance spectrometers made from a printed circuit board having an electrically conductive surface printed on one side thereof and folded so as to form a plurality of the walls of a parallelepipedon cavity resonator, the conductive surface of the circuit board forming the inner conductive surfaces of the cavity. Modulation coils formed by printed circuit techniques on the other side of the circuit board serve as field modulation coils for the cavity resonator, the conductive inner wall surfaces aligned with said coils being made pervious to enhance penetration of the modulation field into the cavity resonator.

United States Patent Hansen I Y [151 3,701,959 [451 Oct. 31, 1972 Arnold et a1 ..324/.5 AH

3,197,692 7/1965 l-lyde ..324/.5 AC

3,205,432 9/1965 Cochran ..324/58.5 C 9/1969 Bull ..313/76 X Primary Examiner-Herman Karl Saalbach Assistant Examiner-Wm. 1-1. Punter Attorney-Stanley Z. Cole and Vincent W. Cleary 5-7] ABSTRACT A microwave cavity resonator useful in electron paramagnetic resonance spectrometers made from a printed circuit board having an electrically conductive surface printed on one side thereof and folded so as to form a plurality of the walls of a parallelepipedon 3 Claims, 6 Drawing Figures [72] Inventor: Stuart C. Hansen, Santa Clara County, Calif. 73 Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed; March 24, 1971 [21] App1.No.; 127,745

[52] "us. c|..... ..333/83 R, 29/601 [51] Int. Cl. ..H01p 7/06, 1101p 1 1/00 [58] Field of Search ..333/83 R, 29; 29/600-601; 324/585 C, .5 AC, .5 AH

[56] References Cited 3 UNITED STATES PATENTS 3,195,079 7/1965 Burton et a1. ..29/600 X 2,262,020 11/1941 Llewellyn ..333/83 R 3,331,017 7/1967 Gielow..... ..324/58.5 C X 2,636,125 4/1953 Southworth ..,....29/600 3,122,703 2/1964 Rempel etal ..324 /.5 R

PATENTEDncraHQR 3.701.959-

FIG.2

NTOR.

STUART c. NSEN n ATTORNEY)" MICROWAVE CAVITY RESONATOR WITH PRINTED CIRCUIT INTERIOR WALLS AND MODULATION COILS BACKGROUND OF THE INVENTION Certain present day scientific instruments, such as electron paramagnetic resonance spectrometers, utilize microwave cavity resonators to apply the necessary microwave energy to the sample under analysis to produce the desired electron resonance in the matter. The sample is placed within the cavity resonator which is generally mounted on one arm of a microwave bridge,an iris opening in the cavity permitting the microwave energy'to pass into the resonator from the bridge. The cavity is placed in the gap of a magnet which provides the necessary polarizing magnetic field for the sample. Two modulation coils are mounted on opposite sides of the cavity resonator to permit a low frequency modulation of the polarizing magnetic field to periodically sweep through the point of maximum electron resonance.

The precision requirements of such cavity resonators have resulted in expensive manufacturing techniques, including machining the cavity resonator and hand winding the modulation coils affixed to the sides thereof. The cost of manufacturing the cavity resonator structure has contributed significantly to the end price of the spectrometer, which places the instrument beyond the financial means of many educational institutions.

Also, even though manufactured by precision techniques, such cavity resonators exhibit certain problems in 'use. For example, the modulation coils mounted on the sides of the cavity are hand wound and consist of many turns of electrically insulated wire, with the individual windings side-by-side and also stacked one level upon another.

The coil is then made integral with and affixed to the cavity side wall by a suitable glue such as epoxy. In operation, repulsive forces are set up between the unidirectional magnetic field produced by the magnet in which the cavity resonator is immersed and the alternating current passing through the modulation coils. This force extends in the direction of the plane of the coil, i.e. parallel to the surface of the cavity wall on which the coil is affixed, and alternates in direction at the frequency of modulation. The force thus tends to shear the windings from the wall surface and from each other, thus setting up vibrations in the coil windings and cavity walls, producing an undersired spurious effect called potatoe."

BRIEF SUMMARY OF THE INVENTION The present invention provides a microwave cavity the upper side, -by a layer of electrically conductive material, the board being folded so that a plurality of the inner walls of a parallelepipedon cavity resonator are formed by said conductive surface of the circuit board.

In one embodiment, the two side walls and the top and bottom walls of a cavity resonator are formed by folding the circuit board along three folds which form three of the juncturesof the inner walls, the fourth juncture being formed by an electrically conducting seam. The two end walls may be formed by suitable electrically conducting end plates.

The two modulation coils are formed by printed circuit techniques on the under surface of the circuit board and in alignment with the two side wall segments on the upper surface, such that, with the board folded toform the cavity, the modulation coils are positioned on the outer surfaces of the two side walls of the cavity. Theinner side-wall surfaces are made suitably pervious such that the modulation magnetic field may penetrate into the cavity while substantial current paths are retained on the inner wall surfaces for the How of microwave current. i

The single layer modulation coils formed by the printed circuit techniques are very strong in shear due to the tenacity of the bond of each winding to the circuit board, and the vibrations which normally produce the undesired potatoe effect are effectively eliminated.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of the under surface of the printed circuit board prior to folding into a cavity resonator configuration.

FIG. 2 is a plan view of the upper surface of the cir cuit board of FIG. 1. V

' FIG. 3 is a perspective view of the completed cavity resonator formed by the folded circuit board with end walls affixedthereto.

A FIG. 4 is a cross-section view of the cavity resonator taken along section line 4-4 of FIG. 3.

' FIG. 5 is an enlarged plan view of a portion of the mesh area of the cavity side wall taken at section line 55 of FIG. 2.

FIG. 6 is a cross-section of the wall portion of FIG. 5 taken along section line 6-6.

DESCRIPTION OF THE PREFERRED EMBODIMENT The novel cavity resonator of the present invention is formed from a printed circuit board (of well known construction) manufactured by standard photographic techniques. The board is a glass epoxy sheet or base ll which is one-sixteenth inch thick with 2 A mil thick copper coating for the circuit elements, i.e. 2 02. copper clad, with a gold flash surface.

The conductive coating 12 on the upper surface of board 11 covers the entire surface, except for two sections 13 and 14 where a plurality of small spaced-apart openings exist in the coat, described more fully below.

The under surface of board 11 carries a number of plated areas including two areas 15 and 16 extending along the edges of the two long sides of the board and two coil windings l7 and 18 spaced-apart along the length of the board and approximately'midway across the board. Two rectangular-shaped areas 21 and 22 are aligned with the coils l7 and 18, one area 21 being positioned between the coils and the other area located on the other side of coil 18. A number of additional small pads 23 are provided on the surface. I

' Holes 24, through the board are located in each of the two rectangular-shaped pad areas 21, 22, respectively.

To form the cavity, the under surface of the board 11 is scored or cut along the three lines 26, 27, 28 extending across the width of the surface and between the coils and pads, and the board folded along these lines with the upper coated surface 12 on the inner side of the folded structure. The coating 12 remains intact or integral at the three folds and serves as a hinge for the wall sections. The fourth juncture 29 formed where the two ends of the board are brought together is closed with a solder seam of good electrically conducting material.

The folded sheet 11 thusforms a four walled box structure with open ends and having two side walls 31 and 32 and top and bottom walls 33 and 34 respective- A pair of cup-shaped end plates 35 and 36 of electrically conductive material are sealed in and close the ends of the cavity structure. The base of a U-shaped clip member 37 is affixed to one end plate 35, an iris opening 38 extending through the base and the end plate to communicate with the interior of the cavity. The legs of the clip member 37 are arranged to slip into the open end of a wave guide 39 to couple the cavity resonator to the microwave bridge circuit.

The side walls 31 and 32 carry the coils 17 and 18, respectively, utilized as the modulation coils to modulate the magnetic field extending normal to these side walls when the cavity is placed in the magnet of the system.

Tubular members 41 and 42 are affixed to the two pads 21 and 22 over the openings therein to permit the sample under investigation and carried in a vial to be inserted into the cavity resonator.

Electrical connections with the transmission line supplying the modulation signal to the coils 17, 18 are made with the terminal pads 43 of the two coils. The entire cavity structure is then encased in a suitable insulating potting material 44. The coated areas 15, 16 and 23 serve to enhance the adhesion of the insulating encasing to the cavity walls.

The small holes 45, e.g. 0.030-inch diameter, in areas 13 and 14 on the inner surfaces of the two side walls and aligned with the modulation coils 17, 18 provide a pervious mesh surface which allows the modulation fields to penetrate into the cavity, thus improving the modulation substantially. The porosity of the mesh areas 13, 14 is established at about 40 percent so that sufficient conductive surface is provided around the openings 44 to give a good current path for the microwave currents. The circumferential resistance of each of the holes 45 is maintained sufficiently high so thatthe small bucking magnetic fields established by the eddy currents set up in the surface around the holes do not significantly reduce the strength of the polarizing magnetic field passing through the mesh.

The single level winding of the modulation coil affixed to the circuit board surface is very strong in the direction of shear, i.e. parallel to the wall surface, because of the strong adhesion of the printed circuit to the board. The coils l7, 18 therefore withstand the forces applied thereto, which tend to move the windingtsJ sideways, b interaction of the magnet fields set up y the modu ating current and the magnets field. The potatoe effect produced by hand wound, multilayer coils. is substantially eliminated by the use of these printed circuit coils.

What is claimed is:

1. A microwave cavity resonator comprising a para]- lelepipedon cavity including two side walls, a top and bottom wall and two end walls having electrically conducting inner wall surfaces, at least four of said walls being formed by a unitary sheet of printed circuit board having one surface coated with an electrically conductive film which forms said electrically conducting inner wall surfaces, said circuit board being folded along a plurality of lines each fold forming the juncture of a different two of said inner wall surfaces wherein two of said four walls comprise said two side walls, said inner electrically conducting wall surfaces on said two side walls having a portion thereof previous to magnetic flux, and a modulation coil formed by a printed circuit on the outer surface of each of two side walls aligned with said pervious portions of the inner wall surfaces.

2. A microwave cavity as claimed in claim 1 wherein said pervious portions are formed by a plurality of spaced-apart openings extending through the electrically conductive surface and into the printed circuit board.

3. A microwave cavity as claimed in claim 2 wherein said openings occupy less than half of the surface area of said pervious portions of the side walls. 

1. A microwave cavity resonator comprising a parallelepipedon cavity including two side walls, a top and bottom wall and two end walls having electrically conducting inner wall surfaces, at least four of said walls being formed by a unitary sheet of printed circuit board having one surface coated with an electrically conductive film which forms said electrically conducting inner wall surfaces, said circuit board being folded along a plurality of lines each fold forming the juncture of a different two of said inner wall surfaces wherein two of said four walls comprise said two side walls, said inner electrically conducting wall surfaces on said two side walls having a portion thereof previous to magnetic flux, and a modulation coil formed by a printed circuit on the outer surface of each of two side walls aligned with said pervious portions of the inner wall surfaces.
 2. A microwave cavity as claimed in claim 1 wherein said pervious portions are forMed by a plurality of spaced-apart openings extending through the electrically conductive surface and into the printed circuit board.
 3. A microwave cavity as claimed in claim 2 wherein said openings occupy less than half of the surface area of said pervious portions of the side walls. 